In recent years, in the field of flexographic printing, a Computer to Plate Technique (CTP Technique), known as a digital image-forming technique, has been widely used as a general technique. The CTP technique is a method in which information processed on a computer is directly outputted onto a printing plate so that a concave/convex pattern to form a relief is obtained. This technique eliminates the necessity of negative film producing processes, thereby making it possible to reduce costs and time required for forming the negative film.
In the CTP technique, the negative film for covering areas that should not be polymerized by exposure is replaced by a mask that is formed within a printing plate.
Also, in Patent Document 1, a method has been proposed where an infrared sensitive layer that is opaque to chemical rays is formed on a photosensitive resin layer and by evaporating this infrared sensitive layer by using an infrared laser, an image mask is formed. This photosensitive printing original plate is constituted from photosensitive resin layer, protective layer and infrared sensitive layer. The protective layer has a function of preventing the mass transfer between the photosensitive resin layer and the infrared-sensitive layer and of preventing the polymerization inhibition of the photosensitive resin layer due to oxygen in the air.
On the other hand, as to the photosensitive resin layer, that which is developable by an aqueous type solution has been strongly demanded in view of affection to the environment and of workability. As to the flexographic photosensitive resin layer developable by water, that comprising synthetic rubber and hydrophilic polymer has been put into the market (cf. Patent Document 2). Moreover, in recent years, there has been developed a water-developable plate where resolution is further enhanced by the use of latex which is present in fine particles as a main component (cf. Patent Document 3).
However, when the photosensitive resin layer as such which is developable by water is used as a photosensitive layer for CTP plate, there is a problem that wrinkles are generated on the plate surface since the flexibility of the photosensitive resin is high. The wrinkles are generated when the plate is detached to return to the flat plane after the plate is attached to a drum in a laser processing. The above problem is particularly significant when a photosensitive resin layer comprising the latex as a main component is used. It is presumably because the latex is present in fine particles whereby the plate is more flexible.
As to a method for preventing the wrinkles on the plate, there has been proposed a method where a protective layer on the photosensitive resin layer is omitted so that the photosensitive resin layer directly contacts with the heat-sensitive mask layer (cf. Patent Document 4). However, when this method is applied to a water-developable plate comprising the latex as a main component, the polymerization inhibition of the photosensitive resin layer due to oxygen in the air is vigorous and satisfactory plate is never achieved. There is also proposed a method where the elastic modulus of the protective layer on the photosensitive resin layer is lowered (cf. Patent Document 5). In this method, large amount of plasticizer is compounded with the protective layer to lower the elastic modulus. Although this method is effective for the prevention of wrinkles, heat resistance of the protective layer lowers by compounding with large amount of the plasticizer, and the protective layer is damaged by the heat upon laser ablation. Therefore, there is a tendency that the plate surface abnormality such as concaves or scanning lines is apt to be generated. As such, it is the current status that no completely satisfactory plate has been achieved in a water-developable CTP plate or, particularly, a water-developable CTP plate comprising the latex as a main component.